for me it's SIMS... in SIMS you can do whatever you want, you can create your own family, build your dream house, control your relationship. StarCraft and Red Alert are also good. right now I'm addicted to playing NBA Live

My favourite pc game of all time that is a single player game is Mafia. Its brilliant from start to finish, you play the role of a taxi driver turned gangster and the storyline in the game is better than any other game before or after it, its superb.
My favourite online game is Soldier of Fortune 2. Its one of the older first person shooters but its still the best in my opinion. Its simple and yet exciting to play.